In the related art, for example, a rubber composition used for a tire is required to balance grip performance (wet grip performance) on a wet road surface and rolling resistance performance that contributes to low fuel consumption at a high level. However, since these are conflicting characteristics, it is not easy to improve these two characteristics at the same time.
PTL 1 suggests that a copolymer resin of a C5 fraction due to thermal decomposition of naphtha and styrene or vinyl toluene compounded in order to improve the wet grip performance without degrading the rolling resistance performance. In this case, the wet grip performance can be improved, but there is a problem of a decrease in operation stability due to degradation of the hardness of a rubber composition at normal temperature. Further, since the modulus of elasticity of the rubber composition at a low temperature is increased so that the grip performance is degraded, low temperature performance becomes problematic.
PTL 2 discloses blending of a low-molecular-weight vinyl aromatic diene copolymer (styrene-butadiene liquid polymer) which has a molecular weight of 2000 to 50000 as a rubber component for the propose of providing a rubber composition having excellent road surface gripping force, fracture strength, abrasion resistance, and heat resistance. In this case, degradation in hardness of a rubber composition to be obtained is significant, and the operation stability when the rubber composition is applied to a tire is decreased.
PTL 3 discloses blending of a (meth)acrylate polymer which has a weight average molecular weight of 5000 to 1000000 and a glass transition point of −70° C. to 0° C. for the purpose of suppressing degradation of low temperature performance and rolling resistance performance and improving wet grip performance. However, blending of a particulate (meth)acrylate polymer having a specific particle diameter is not disclosed.
In addition, a technique of blending rubber gel which is a cross-linked rubber particle with a rubber composition comprising diene rubber has been known. For example, PTL 4 discloses that modified diene rubber, cross-linked rubber particles, and silica are kneaded in order to obtain a rubber elastic body having lower rolling resistance and excellent rebound resilience. In PTL 4, while dispersibility of silica is increased by modified diene rubber, silica is partially unevenly distributed by excluding silica using cross-linked rubber particles. Further, PTL 5 discloses that both of wet grip performance and rolling resistance performance are achieved by blending styrene-butadiene rubber gel. However, the wet grip performance is not highly improved while suppressing degradation of hardness at normal temperature and an increase in modulus of elasticity at a low temperature.
PTL 6 discloses blending of (meth)acrylic acid alkyl ester polymer particles with a rubber composition. However, in PTL 6, the particles contribute to improvement of on-ice friction resistance by forming micro irregularities on a tread surface of a studless tire. Therefore, it is necessary to use particles having a particle diameter of 0.1 μm to 100 μm and preferably 1 μm to 30 μm, which is relatively large. It is not disclosed that the wet grip performance can be improved while suppressing degradation of hardness at normal temperature and an increase in modulus of elasticity at a low temperature by blending a particulate (meth)acrylate polymer having a smaller particle diameter.
PTL 7 discloses blending of nano particles of a non-aromatic vinyl polymer (for example, a polymer of (meth)acrylate) containing a reactive silyl group represented by formula ≡Si—X (in the formula, X represents a hydroxyl group or a hydrolyzable group) with a rubber composition. However, in PTL 7, it is essential that the nano particles contain a reactive silyl group because the nano particles are used as a reinforcing filler and exhibit reinforcing properties by being combined with a coupling agent. It is not disclosed that the wet grip performance can be improved while suppressing degradation of hardness at normal temperature and an increase in modulus of elasticity at a low temperature by using fine particles formed of a (meth)acrylate polymer that does not contain a reactive silyl group.
Meanwhile, PTL 8 discloses that a branched conjugated diene copolymer obtained by introducing a branched conjugated diene component such as myrcene into a molecular chain of styrene-butadiene rubber is used as a rubber component of a rubber composition for a tire. However, in PTL 8, the branched conjugated diene compound is introduced as a constitutional unit of a copolymer for the purpose of improving the characteristics of styrene-butadiene rubber having a large amount of styrene. It is not easy to suppress degradation of low temperature characteristics because the copolymer is used as a rubber component.